Examinations or treatments of an (ill) patient are increasingly carried out in a minimally invasive manner, i.e. with as little operative effort as possible. Examples of such procedures are treatments using instruments such as endoscopes, laparoscopes, catheters or biopsy needles, which are inserted into the examination area of the patient, via a small opening in the body in each instance. By way of example, catheters are frequently used within the scope of angiographic or cardiological examinations.
To navigate these instruments (e.g. in the head or the heart), so-called intraoperative 2D images (real-time images) are obtained during the intervention on the basis of fluoroscopic or angiographic fluoroscopy (e.g. with the C-arm). Such an angiography is shown for instance in FIG. 1.
The problem from a medical-technology perspective lies in these fluoroscopy images indicating no spatial (three-dimensional) details compared with 3D angio images.
However, these fluoroscopy images are available quickly and minimize the radiation exposure for patient and doctor.
In the prior art, the spatial information is hereby regained in that preoperatively recorded 3D images (for instance from a CT tomography, 3D angio tomography or an MR tomography) are registered with the two-dimensional intraoperative fluoroscopy images and are underlaid with said images, which is referred to as co-registration.
A co-registration method of medical images of different imaging modalities is disclosed in U.S. Pat. No. 5,672,877 for instance. An emission image (e.g. a SPECT image, acronym for “Single-Photon Emission Computed Tomography”) recorded using a nuclear-medical imaging system) is acquired, which is registered with a transmission image (e.g. a CT image, recorded with the same or another imaging system) so as to display functional structures and anatomical structures similarly in a single image, overlaid for instance.
U.S. Pat. No. 6,019,724 A discloses a method, in which intraoperative 2D and/or 3D (ultrasound images), which map the respective current position of the used surgical instrument, are overlaid on the preoperative images of other imaging modalities (MR, CT, C-arm etc.) in order to assist the surgeon in guiding the instrument.
The combination of such co-registered 2D and 3D images now allows the doctor a better orientation in the relevant volume range.
Such 2D-3D co-registration according to the prior art comprises two steps:    1. the image registration itself and    2. the visualization.
With image registration, it must first be determined from which direction a 3D volume must be projected so that it can be made congruent with the intraoperative 2D image and a suitable second 2D image must be generated from the 3D image data set, said second 2D image being able to be made congruent with the first intraoperative 2D image. This second 2D image is either generated by projecting the 3D image data set or by a clip plane, which is placed through the 3D image data set. This must determine from which direction the projection has to be carried out and/or how the clip plane must be placed. Different approaches exist for this, which are however not carried out.
Two standard methods exist inter alia for the visualization of the registered images, in other words the common display of the first 2D image and 3D projection and/or second 2D image or clip plane:    1. overlay and    2. linked cursor.
With the “overlay”, the two images are placed one over the other (with the aid of different methods). The part which each of the two individual images on the overlaid (merged) image is to exhibit can be adjusted and is referred to as “blending”.
US 2004/0013290 A1 discloses a method, which solves the problem of overlaying several (preferably two) 2D and/or 3D images of different imaging modalities, with the aim of merging several different diagnostically relevant structures/information into one single image and making their cohesions visible. The overlay of 2D or 3D outputs of CT or PET imaging modalities is thus kept in mind for instance. Besides other technologies, the overlaid images are also merged by means of “blending technologies”.
With the “linked cursor”, both images are displayed in separate (pop-up) windows and where possible even on separate monitors, with both windows having a common cursor.
Movements in the 2D window are now transferred into the 3D window and vice versa.
The overlay displays the most current method, but is however disadvantageous in that certain low-contrast objects in the 2D image (e.g. catheter tips, stents etc.) can be covered by the high contrast 3D recordings under some circumstances when overlaying and/or cross fading.
This problem does not exist with the linked cursor. The linked cursor is however disadvantageous in its being directed onto two windows (two monitors). Particularly in the operating theatre, only one pop up window is wanted on only one monitor and with the largest possible display.